A coating is a clear over-print applied to a press sheet to improve the overall glossiness and provide abrasion resistance to the printed image. Today's printing industry demands high gloss, high abrasion resistant coatings, for example as dust jackets for books, for advertising brochures, and food packaging.
Coatings are applied in-line on-press as an extra press unit, off-line on coating or varnishing machines, or in-line followed by off-line to achieve the greatest effect. Coatings can be applied locally to selected image areas with conventional offset lithographic techniques on-press, or with blanket cut-outs on coating machines. Coatings may also be applied over the entire sheet with offset blankets either on-press or on a coating machine.
The term "varnishing" is still used to define in-line coating using printing units to apply a coating to a localized area. Occasionally, the term is also used to differentiate creative, special-effect coating from production coating. Varnishing originally referred to coating with a boiled linseed oil varnish, applied on-press as if the varnish were a clear ink. Although varnish increased gloss and rub-resistance to an extent, anti-setoff powder used in printing decreased the gloss and rub-resistance. In some cases, varnish actually decreased gloss.
In general, there are two types of coatings presently used by the printing industry, so called "aqueous" and "UV" coatings, neither of which has proven wholly satisfactory. However, aqueous (acrylic) and ultraviolet (UV) coatings have over the years come to replace varnish as the primary coating mediums for over-printing.
An aqueous (acrylic) coating is a water-soluble, thermoplastic polymer coating that requires heat and air to dry or harden the coating. Aqueous coatings are 60% solvent (water and alcohol) much of which escapes in the coating and drying process, along with some of the gloss. Once applied and dried under infrared heat, aqueous coatings harden instantly, but because they are thermoplastics they can be softened with reapplication of heat.
A UV coating is a monomer coating that requires ultraviolet energy to dry or harden the coating. UV coatings have a photo-initiator which start the ultraviolet hardening process. UV coatings are solventless, which means that once applied and dried under ultraviolet energy UV coatings harden instantly and are not susceptible to softening.
Although aqueous and UV coatings have improved the level of gloss and abrasion resistance over previously used coatings, acrylic and UV coatings are not without problems. For example., aqueous coatings will not adhere to a previous UV coating. Also aqueous coatings are less glossy and less rub-resistant than UV coatings. Aqueous coatings have a tendency to "reactivate" on the sheet, for example, when stored in a warm warehouse, which can cause the sheets to stick together and ruin the job.
UV coatings are more expensive than aqueous coatings, requiring special expensive equipment and ultraviolet lights, which require workers to wear protective clothing and goggles. UV coatings have a tendency to crack when a printed sheet is folded, and cannot be printed over once applied to a substrate. Also, UV coatings typically require special catalysts and other additives to impart an acceptable coating.
In general, the prior art coatings are not especially useful in many different applications for which coatings are required, such as hot- and cold-food packaging. Often, coatings of the prior art exhibit low gloss, poor scratch resistance, poor waterproofing, heat and cold sensitivity, and adherence problems relative to the substrate. Frequently the coatings of the prior art do not significantly minimize the tendency for ink colors to fade. Also, these prior art coatings do little to improve the otherwise dull colors of ink printed on recycled paper.
Accordingly, it would be useful to develop a coating for printed substrates that eliminated or reduced most or all of the above-noted problems experienced with the prior art coatings.
It is an object of the present invention to provide a non-aqueous coating that is waterproof and highly abrasion resistant.
It is a further object of the present invention to provide a coating that imparts improved gloss to a substrate.
It is yet another object of the present invention to provide a coating which protects against fading of colors over which the coating is applied.
It is another object of the present invention to provide a coating which is insensitive to hot and cold temperature extremes.
It is still another object of the present invention to provide a coating which brightens ink or other pigments over which the coating is applied.
It is a further object of the present invention to allow coating and/or printing over surfaces which normally cannot be adhered to.
It is another object of the present invention to provide a coating which is quick-drying.